The invention is based on a fuel injection pump as generally described by the preamble to the main claim. A fuel injection pump of this type is already known from German Pat. No. 11 72 897, the impact protection ring of which is drawn from sheet steel and is pressed by a snap ring, acting as a holder means, against an end face of a holder bushing embodied as an abutment shoulder. Given the previously conventional injection pressures, such a protection ring was completely satisfactory, and the hardened surface of the ring resisted the impact energy of the fuel stream leaving the return flow opening at the end of injection. However, it has been found that with the increase in injection pressures required for modern direct-injection engines, the impact protection rings previously used are destroyed in the course of time; the fuel stream which exits with extremely high energy wears off the surface of the inner wall of the impact protection ring by erosion until the wall has been broken through. Subsequently, the pump housing, which is normally of aluminum, is destroyed very rapidly by the fuel stream, which is no longer hindered by the protection ring.
Longer service life of the impact protection ring and thus a longer life of the injection pumps can be attained if the impact protection ring executes a rotary movement during operation, so that new wall locations are continually being presented to the fuel stream. Such a rotary movement was demonstrated in some cases, but in the known rings it took place only occasionally; a rotary movement which was made automatic by appropriate means and which took place under all operating conditions was not previously attainable.
From German Offenlegungsschrift No. 24 42 010 and German Offenlegungsschrift No. 24 42 088, sheath-like impact protection rings have become known which were built in with play both in the axial and the radial directions in order to enable a rotary movement. In these fuel injection pumps, the danger exists that oscillations on the part of the impact protection rings caused by the existing play will destroy the adjacent abutment faces and also the end faces of the rings very rapidly; a reliable rotary movement occurring under all conditions could not be observed. In the slotted embodiment of the known ring according to German Offenlegungsschrift No. 24 42 010, the danger exists that the gap existing at the connecting point of the ring presents a face which the fuel stream can act upon, and the gap experiences the flow through it of fuel.